We plan to characterize plasmids which have been constructed in our laboratory and which are thought to carry cysM and cysB genes from Salmonella typhimurium. Strains carrying cysB plasmids will be assayed for overproduction of the cysB regulatory protein. The cysB protein may be purified from such strains. We shall continue in our efforts to determine whether an impairment in transmethylation contributes to the pathogenesis of the immune defect in patients with heritable deficiency of adenosine deaminase.